Dual-stage-pole-piece and multiple-stage-pole-piece ferrofluid or magnetic seal apparatuses are well known and are designed to provide a seal typically about a rotatable shaft between two different environments, such as high- and low-pressure environments, for multiple-stage ferrofluid seals, or to provide an exclusion seal to exclude one environment from altering another environment. In a ferrofluid exclusion seal, the function of the ferrofluid seal is to exclude contaminants from reaching a sensitive part of a device or an environment protected by the seal. Typically, such exclusion seals are not required to provide any particular pressure capacity between environments of high and low pressure. Examples of the use of typical exclusion seals are the use of dual pole-piece ferrofluid seals with computer-disc-drive spindles, in order to prevent particulate contaminants from reaching or contacting the memory disc area of the computer and wiping out or affecting the stored memory, the sealing of motor shafts or textile spindles in the textile industry to prevent spindle or motor-bearing failings caused by fibrous materials, and to protect sensitive instruments from undesirable environments, such as from salt-spray mist in ocean environments or prevent corrosive chemical environments from reaching sensitive equipment. In such exclusion ferrofluid seal applications, the pressure capacity required for the ferrofluid exclusion seal is normally very small, and typically, for example, about 0.5 of a pound or less.
Ferrofluid-sealing technology is well known with the use of ferrofluid-type exclusion seals. The typical design of the exclusion seal consists of a permanent magnet and two magnetic permeable pole pieces, with the magnetic-flux circuit completed through the magnetically permeable shaft, which is usually provided by the seal customer. If desired, each pole piece may consist of one or more stages, in order to increase the pressure capacity of the seals and, thus, form a multiple-stage exclusion pressure-type seal, such as described in U.S. Pat. No. 3,620,584. The ferrofluid or the magnetic liquid, which may have an aqueous, hydrocarbon or other liquid carrier, and typically comprises a non-volatile carrier, is retained under each particular stage by the magnetic flux lines and forms a ferrofluid O-ring about the surface of the shaft. The low vapor pressure of the ferrofluid usually ensures a long maintenance-free seal life.
It is desirable, particularly in computer-disc-drive spindle applications and in similar applications, to provide an improved ferrofluid exclusion seal operation and exclusion seal system of simple design, low cost and small space requirements.